1. Field of the Invention
This invention relates to a method of controlling the feed rate of a cutter, and to the associated apparatus. In particular, the invention is directed to a cutter feed rate control method and apparatus suited for a machine tool in which an electric motor having operating regions of constant torque and constant output characteristics is employed for driving a spindle such that the machine tool can cut a workpiece while the circumferential speed of the workpiece is held constant by the motor.
2. Description of the Prior Art
In machine tools, particularly lathes, a cutting operation is performed while the circumferential speed of a workpiece is controlled so as to remain constant. Cutting a workpiece through this method of circumferential speed control affords the following advantages:
(1) The smoothness and luster of the workpiece surface are enhanced;
(2) Machining time (cutting) is shortened; and
(3) Cutting quantity can be held constant.
The output power P [KW] of a spindle motor employed in a machine tool such as a lathe rises linearly until a specified speed (referred to as the base speed) Nb is attained, and then a constant maximum value P.sub.max thereafter is kept. The cutting quantity Q is dependent upon the spindle motor output power P. Hence, the cutting quantity Q can be maximized and held constant at a maximum value Q.sub.max at the base speed N.sub.b or above, but decreases in accordance with its dependence upon speed for values less than N.sub.b.
In control systems for maintaining a constant circumferential speed, the greater the outer diameter of a workpiece, the lower the motor or spindle speed N.sub.s, the latter falling below the base speed N.sub.b if the outer diameter of the workpiece is greater than a certain value. Accordingly, when machining a workpiece having a large outer diameter with the prior-art control systems for maintaining a constant circumferential speed and, hence, a constant cutting quantity, it is conventional practice not to perform cutting at the maximum cutting quantity Q.sub.max based on the maximum output of the spindle motor, but to do so after deriving a certain cutting quantity Q. This is accomplished by first obtaining the minimum spindle speed which is decided by the maximum outer diameter D.sub.max of the workpiece, calculating the motor output power P (less than P.sub.max) which conforms to the minimum spindle speed, and then finally obtaining the cutting quantity Q (less than Q.sub.max) as decided by this output P. The cutting operation is then a matter of determining a cutter feed rate f.sub.r so as to satisfy the cutting quantity Q, and feeding the cutter in accordance with this feed rate f.sub.r.
However, with this conventional cutting method the motor or spindle speed N.sub.s eventually surpasses the base speed N.sub.b owing to a gradual decrease in workpiece diameter as machining progresses. The cutting conditions may thus become such as will allow cutting to advance at the maximum cutting quantity Q.sub.max. The foregoing method nevertheless constrains machining to proceed at the constant cutting quantity Q. As a result, cutting in accordance with the conventional method of circumferential speed control is beset by problems since it does not have the merit of constant circumferential speed control, nor permit full exploitation of spindle motor output power, nor a satisfactory shortening of machining time.